The overall objective of these studies is to further our understanding of how caldesmon contributes to the regulation of smooth muscle and non muscle actomyosins. The conceptual framework is a qualitative model that describes the coordinated interaction of the four putative regulatory proteins: 1) myosin regulatory light chain (LC20) 2) tropomyosin, 3) calponin, and 4) caldesmon. The model is based on a simple two-state kinetic scheme, and incorporates the allostericIcooperative-activation model for thin filament regulation proposed by Eisenberg and Hill (27). The proposed studies will address the mechanism by which caldesmon interacts with tropomyosin, calponin, and the regulatory light chain of myosin to modulate contraction. Specifically, these studies will test the hypothesis that caldesmon inhibits the "switching-on" of actin filaments, and thereby inhibits activation of dephosphorylated cross bridges. Our general approach to investigate the specific effects of tropomyosin and caldesmon on the actomyosin interaction will be to utilize purified proteins and recombinant peptides to reconstitute regulated actin filaments. We will employ an in vitro motility assay to measure unloaded thin-filament velocity. A recent modification of the motility assay allows us to also rapidly measure the force exerted on individual actin filaments under steady-state isometric conditions. These studies should yield new insights into the molecular mechanisms that govern actomyosin-based contraction in both smooth muscles and nonmuscle cells. As such these findings may be relevant not only to vascular disease states involving abnormal smooth muscle contraction, such as hypertension, but also those involving inappropriate cellular growth and proliferation such as atherosclerosis.